The autonomic nervous system has two components, including a sympathetic component that is relatively slow acting, and a parasympathetic component (also known as the vagal component) that provides a relatively faster response than the sympathetic component. A proper balance between the sympathetic and parasympathetic components of the autonomic nervous system is believed to be important. Accordingly, an indication of the balance of these components of the autonomic nervous system, which is sometimes referred to as “autonomic balance” or “sympathovagal balance”, is sometimes used an indication of a patient's well-being.
Quantitative assessment of the autonomic nervous system's activity has largely been based on analysis of heart rate variability (HRV). In such techniques, a measure of sympathovagal balance is often given in terms of some form of standard deviation in RR intervals (e.g., SDNN, a time domain approach) or ratio of low to high frequency components of the power spectrum (the spectral approach).
More specifically, in one time domain approach, the standard deviation of RR intervals (SDNN) is measured. In this approach, an increase in SDNN is interpreted as an increased predominance of the sympathetic component (and a proportional decrease in the parasympathetic component), where a decrease in the SDNN is interpreted as an increased predominance of the parasympathetic component (and a proportional decrease in the parasympathetic component).
In one spectral approach, measures of normal RR intervals are converted into the frequency-domain so that its spectral frequency components can be analyzed. Two frequency bands are indicated as being of interest, including, e.g., a low frequency (LF) band (e.g., between 0.04 Hz and 0.14 Hz) and a high frequency (HF) band (e.g., between 0.15 Hz. and 0.40 Hz). The HF band of the R-R interval signal is believed to be influenced by only the parasympathetic component of the autonomic nervous system. The LF band of the R-R interval signal is believed to be influenced by both the sympathetic and parasympathetic components of the autonomic nervous system. Consequently, the ratio LF/HF is used as an indication of the autonomic balance between sympathetic and parasympathetic components of the autonomic nervous system. More specifically, an increase in the LF/HF ratio is interpreted as an increased predominance of the sympathetic component (and a proportional decrease in the parasympathetic component), where a decrease in the LF/HF ratio is interpreted as an increased predominance of the parasympathetic component (and a proportional decrease in the parasympathetic component).
A related time domain approach obtains a first measure that is believed to be influenced by only the parasympathetic component of the autonomic nervous system, and a second measure that is believed to be influenced by both the sympathetic and parasympathetic components of the autonomic nervous system. As in the above described spectral approach, a ratio is then taken of the two measures to obtain a measure of the autonomic balance between sympathetic and parasympathetic components of the autonomic nervous system. Similarly, an increase in the ratio is interpreted as an increased predominance of the sympathetic component (and a proportional decrease in the parasympathetic component), where a decrease in the ratio is interpreted as an increased predominance of the parasympathetic component (and a proportional decrease in the parasympathetic component).
Though useful in quantifying the sympathetic and parasympathetic balance, both frequency and time domain approaches that have been suggested do not provide information on the sympathetic and parasympathetic components independently. For example, decreased HRV can be described by either a decreased SDNN or increased LF/HF ratio. However, both of these measures are interpreted as being caused by either a decrease in parasympathetic tone (also referred to as vagal tone) or an increase in sympathetic tone. In other words, conventional approaches for quantifying the sympathetic and parasympathetic balance have assumed that as one component goes up, the other component goes proportionally down, and vice versa.